River Discharge Dynamics Research Articles

Seasonal Study of the Kako River Discharge Dynamics into Harima Nada Using a Coupled Atmospheric–Marine Model

This study developed a coupled atmospheric–marine model using the COAWST model system for the Harima Nada area between spring 2010 and winter 2011 to evaluate the seasonal influence of the Kako River’s discharge in the sea. The Kako River is one of the largest rivers in southwest Japan, contributing almost half of the freshwater discharged in the Harima Nada region in the Seto Inland Sea. Validation was conducted for the entire period, showing a good performance for the atmospheric and marine variables selected. Multiple experiments injecting an inert tracer in the Kako River estuary were performed to simulate the seasonal river water distribution from the estuary into the sea and to analyze the seasonal differences in concentration patterns and mean residence times in Harima Nada. Because the study area is shallow, the results were evaluated at the surface and 10 m depth layers and showed significant seasonal differences in tracer distribution, circulation patterns, and mean residence times for the region. On the other hand, differences seemed to not be significant during the same season at different depths. The obtained results also agreed with the area’s natural water circulation, showing that the Kako River waters tend to distribute towards the west coast of Harima Nada in the warmer seasons but shift towards the east in winter. The influence of the Kako River in the center of the study area is seasonal and strongly dependent on the direction of the horizontal velocities more than their magnitude. The mean residence times varied seasonally from approximately 30 days in spring to 12 days in fall. The magnitude of the horizontal velocity was found to be maximum during summer when circulation patterns at the surface and 10 m depth in the central part of Harima Nada also seem to promote the strongest horizontal and vertical mixes.

Exploring Temporal Dynamics of River Discharge Using Univariate Long Short-Term Memory (LSTM) Recurrent Neural Network at East Branch of Delaware River

River flow prediction is a pivotal task in the field of water resource management during the era of rapid climate change. The highly dynamic and evolving nature of the climatic variables, e.g., precipitation, has a significant impact on the temporal distribution of the river discharge in recent days, making the discharge forecasting even more complicated for diversified water-related issues, e.g., flood prediction and irrigation planning. In order to predict the discharge, various physics-based numerical models are used using numerous hydrologic parameters. Extensive lab-based investigation and calibration are required to reduce the uncertainty involved in those parameters. However, in the age of data-driven predictions, several deep learning algorithms showed satisfactory performance in dealing with sequential data. In this research, Long Short-term Memory (LSTM) neural network regression model is trained using over 80 years of daily data to forecast the discharge time series up to seven days ahead of time. The performance of the model is found satisfactory through the comparison of the predicted data with the observed data, visualization of the distribution of the errors, and R2 value of 0.93 with one day lead time. Higher performance is achieved through the increase in the number of epochs and hyperparameter tuning. This model can be transferred to other locations with proper feature engineering and optimization to perform univariate predictive analysis and potentially be used to perform real-time river discharge prediction.

Wavelet analysis of hydro-climatic time-series and vegetation trends of the Upper Aragón catchment (Central Spanish Pyrenees)

Water managers and researchers noted with concern a nearly generalized decline in Mediterranean rivers discharge over the last decades. Changes in climatic forces (precipitation and air temperature) and land use and land cover (LULC) changes characterized by re-vegetation and greenness are the two most possible explanations for this discharge decline. The direct impact on river discharge stemming from these changes is difficult to assess and their role is generally studied separately. Here, we use the method of wavelet transformation to interpret the time-scale dependency of catchment discharge concerning the uneven temporal climatic fluctuations and re-vegetation processes. We analyzed the temporal variation of concurrent air temperature, precipitation and river discharge time-series for the Upper Aragón catchment, located in the Central Spanish Pyrenees. A long-term database collected over 60 years (1956–2020) was used. Land cover maps corresponding to different decades were used and the results indicated that the catchment experienced a significant increase in the area covered by mixed and broadleaf forests, mostly as a consequence of land abandonment. We show how temperature slightly increased and precipitation moderately decreased. However, catchment discharge experienced a sharp decline in its magnitude and also changes in its temporal variability dynamics. The relevance of the seasonal time-scales with regard to the available discharge is reduced, which strengthens the importance of the inter-annual time-scales for the catchment discharge dynamics. Furthermore, the catchment storage-discharge cycle at inter-annual time-scales is also reduced. Such changes can mostly be attributed to the changes in plant coverage, with an increasing weight in shaping hydrological processes at catchment scale due to the greenness effect. As such, we conclude that LULC changes have played a dominant role on the river discharge dynamics. Climatic trends, on the contrary, have been small, and they have played a secondary role in the decline of river discharge. Future research can use these observations to constrain the pace of upcoming water demands based on the available water resources at Mediterranean catchment scale.

High-resolution satellite images combined with hydrological modeling derive river discharge for headwaters: A step toward discharge estimation in ungauged basins

The reliance on ground-based measurements has been a long-standing barrier toward deriving river discharge for ungauged headwaters, which are quite sensitive to climate change due to relatively high contributions of snow and glacier meltwater to river discharge. In addition, information pertaining to discharge is scarce and inconsistent across the globe due to economic, geographic, and political reasons. Therefore, understanding spatiotemporal dynamics of river discharge in ungauged headwaters is extremely challenging, particularly in high-mountain regions such as the Tibetan Plateau (TP). This study presents a methodology of estimating daily continuous discharge using multisource remote sensing and a hydrological model that can simulate cryospheric processes including snow and glacier accumulation and melt. The hydrological model is forced and calibrated purely by satellite-observed information, particularly by river widths derived from high-spatial-resolution images without using in situ river discharge measurements. Poorly gauged or ungauged headwaters with river widths on the order of 100 m, including the Lhasa (the tributary of the upper Brahmaputra River), Salween, Mekong, and Yangtze rivers originating on the TP were chosen to test the methodology. At-a-section river widths were derived from high-resolution satellite images (i.e., IKONOS, QuickBird, RapidEye-1/2/4/5, GeoEye-1, and WorldView-2) and Landsat archives. As exemplified by the Nash–Sutcliffe efficiency coefficient values (NSE) (up to 0.8 and higher than 0.5 overall), simulated daily continuous discharge was reliable in the four testbeds with different river bathymetry, narrow river widths, and complex terrain. Uncertainties associated with the accuracy, number, and variability of calibration references (i.e., remotely sensed widths) and optimization algorithms were fully discussed. This study highlights the potential of deriving daily continuous discharge without a priori information on river flow, paving the way for discharge estimation in ungauged headwaters globally and providing important implications for satellite-based discharge estimation in narrow rivers across high-mountain regions.

Spatio-temporal dynamics of hydrologic changes in the Himalayan river basins of Nepal using high-resolution hydrological-hydrodynamic modeling

The hydrology of the Himalayan region, known as the water tower of Asia, is undergoing rapid transformations due to climate change and growing human influences, and it is known that this region is one of those most vulnerable to climate change. Numerous studies have examined the changes in the hydrology of Nepal, which includes a significant upstream portion of the Himalayas. However, there is a lack of holistic studies on the spatial–temporal evolution of hydrologic dynamics over the entire nation and over long periods. In this study, we present a comprehensive assessment of the changes in river discharge, flood occurrence, and terrestrial water storage (TWS) across all river basins in Nepal using hydrological-hydrodynamic simulations spanning for 40 years (1979–2018) at ~ 5 km spatial resolution, and downscaled flood attributes at ~ 90 m resolution. The spatio-temporal variations in river discharge and inundation extent are examined through mapping of decadal trends using a quantile analysis method. The results indicate that the dynamics of river discharge has evolved varyingly across different river basins. The evolution pattern within a basin generally agrees with that at the basin outlet, but notable exceptions are found, indicating high hydro-climatic heterogeneity within the basins. The decadal evaluation of flood dynamics over major flooded areas suggests that inundation dynamics is strongly influenced by various flow characteristics, including the timing, duration, and magnitude, and that the evolution of flood dynamics is more complex than that of river discharge. Results indicate that the TWS dynamics over entire Nepal is strongly modulated by the variations in subsurface water storage, and groundwater storage has been in continuous decline (-1.74 cm/year) in the recent decades (2002–2016). This study provides a basis to advance the understanding of long-term hydrologic changes in the Himalayan region with important implications for improved water resources management.

Upstream land-use negatively affects river flow dynamics in the Serengeti National Park

In the Greater Serengeti-Mara ecosystem, with the Serengeti National Park (SNP) at its core, people and wildlife are strongly dependent on water supply that has a strong seasonal and inter-annual variability. The Mara River, the only perennial river in SNP, and a number of small streams originate from outside SNP before flowing through it. In those watersheds increasing grazing pressure from livestock, deforestation, irrigation and other land uses affect river flows in SNP that subsequently have impacts on wildlife. We quantified the changes since the 1970s of river discharge dynamics. We found that the baseflow recession period for the Mbalageti River has remained unchanged at 70 days, which is a natural system inside SNP. By contrast it has decreased from 100 days in the 1970s to 16 days at present for the Mara River, coinciding with increased commercial-scale irrigation in Kenya that extract Mara River water before it reaches SNP. This irrigation will result in zero flow in the river in SNP if the proposed dams in the river in Kenya are built. We observed high flash floods and prolonged periods of zero flows in streams draining livestock grazed watersheds, where severe major erosion prevails that results in gully formation. This eroded sediment is expected to silt and dry out the scattered dry season water holes in SNP, which are an important source of drinkable water for wildlife during the dry season. It appears likely that the future water supply of SNP is at risk, and this has major consequences for its people and wildlife. Ecohydrology-based solutions at the catchment scale are urgently needed to reduce catchment degradation while ensuring sustainable water provision.

Intraseasonal Dynamics of River Discharge and Burned Forest Areas in Siberia

This study demonstrates the dependence between the forest burning rates and abnormal decrease in Siberian river discharges under the conditions of the permafrost zone. Our study area is in Central Siberia and Eastern Siberia/Yakutia. Four rivers (Podkamennaya Tunguska, Lower Tunguska, Aldan, and Viluy) were selected for the study. We analyzed the long-term and seasonal variation of river discharges (archive of The Global Runoff Data Centre for 1939–2015) together with the forest burning dynamics within the river basins (archive of Sukachev Institute of Forest for 1996–2015). We compared the discharges per year with the 77-year average value. Abnormally low levels of discharge constituted 58–78% of the averaged annual rate. An analysis of available chronologies of extreme fire events and relative burned areas (RBAs) showed a high correlation with intra-seasonal data on the runoff minima. The most significant response of river discharges to the wildfire effect was shown for the late summer/autumn season after extreme wildfires during the summer period. The deficit of the runoff was not explained by a low precipitation. Late summer and autumn anomalies of discharge were typical (r = −0.57…−0.77, p < 0.05) for rivers of Central Siberia in seasons of extreme forest burning. The correlation was lower for rivers of Eastern Siberia/Yakutia.

Monitoring and analysis of lowland river discharge

In single-thread channels with a sloping bed and quasi-steady con- ditions, river discharge can often directly be inferred from the water level using a rating curve. Challenges occur when the bed slope becomes negligibly small, or when the discharge becomes highly dynamic such as in the case of tides. This paper discusses recent work on monitoring and analysis of river discharge dynamics under such complicating circumstances. In lowland rivers, backwater has an effect on water levels that extends over a large region for smaller bed slopes. Results from a rainfall-runoff model and discharge measurements in the Mahakam catchment show that when a rating curve is developed in a backwater effected river, it essentially filters the backwater effects out of the discharge series. In coastal lowland plains, marine impacts cannot be ignored. Tidal waves may propagate hundreds of kilometers inland and interact with the river discharge. Acoustic Doppler current profilers are increasingly being employed for discharge monitoring in lowland rivers. A recently developed error model shows that the acoustic range needs to extend beyond the distance from the river bank where the depth mean flow velocity exceeds the cross-section averaged velocity.

Assessing the Impacts of Climate Change on River Discharge Dynamics in Oueme River Basin (Benin, West Africa)

Understanding the impacts of climate change on water resources is of utmost importance to successful water management and further adaptations strategies. The objective of this paper is to assess the impacts of climate change on river discharge dynamics in Oueme River basin in Benin. To this end, this paper used the distribution based scaling approach to improve usability of regional climate model projections for hydrological climate change impacts studies. Hydrological simulations in Bétérou and Bonou sub-catchments of the Oueme River were carried out with a lumped conceptual hydrological model. The main contribution of this paper is to use the hydrological model based on the least action principle (HyMoLAP), which is designed to minimize uncertainties related to the rainfall-runoff process and scaling law, for this assessment. The bias correction approach allows reducing the differences between the observed rainfall and the regional climate model (HIRHAM5 and RCA4) rainfall data. Corrected and raw HIRHAM5 and RCA4 rainfall data were compared with the observed rainfall using Mean Absolute Error (MAE) and Root Mean Square error (RMSE). The results of the bias correction show a decrease in the RMSE and MAE of the raw HIRHAM5 and RCA4 rainfall data of approximately 91% to 98% in both catchments. The results of the simulation indicate that the HyMoLAP is suitable for modelling river discharge in the Oueme River basin. For the future projection based on RCP4.5 scenarios, the projected mean annual river discharge by using HIRHAM5 and RCA4 in Bétérou and Bonou decrease with the magnitude ranging respectively from −25% to −39% and −20% to −37% in the three time horizons 2020s (2011–2040), 2050s (2041–2070) and 2080s (2071–2100), representing the early, middle and late of 21st century. As regards the future projection based on RCP8.5 scenarios, the projected mean annual river discharge by using HIRHAM5 and RCA4 in Bétérou and Bonou decrease with the magnitude ranging respectively from −15% to −34% and −18% to −36% in the three time horizons. The model uncertainties projections indicated that the entire discharge distribution shifted toward more extreme events (such as drought) compared to the baseline period.

Influence of the Long-Range Dependence in Rainfall in Modelling Oueme River Basin (Benin, West Africa)

Nowadays, there is evidence that hydrological processes exhibit long-range dependence (LRD), i.e. power-type decay of autocorrelation also known as the Hurst phenomenon. This means that the stationarity assumption of hydrological time series, which has been widely used in the past, cannot be further advocated. The objective of this paper is to detect the long-range dependence in rainfall in Oueme River basin and to understand how the Hurst coefficient influences the river discharge dynamics. To this end, this paper formulated the Hurst phenomenon that characterized hydrological and other geophysical time series. Then, the fractional generalization of the triple relationship between the fractional Brownian motion, the corresponding stochastic differential equations (SDE) describing the river basin and the deterministic fractional Fokker-Planck equations (FPE) is analysed for the modelling of the river discharge dynamics. This fractional FPE provides an essential tool for the study of the dynamics of the river discharge in Oueme River basin.

Continuous river discharge monitoring with bottom-mounted current profilers at narrow tidal estuaries

The objective of this study is to verify whether accurate and continuous estimates of freshwater discharge at the mouth of a narrow estuary with a single channel can be obtained from a bottom-mounted current profiler (ADCP). The focus is on moderate- to high-discharge events that significantly affect the water circulation corresponding to low river flow conditions. Observations at the Guadiana Estuary (southern Iberia) indicate lateral subtidal flow variability, constant cross-channel area, and quasi-steady response of the axial velocity to discharge events. Based on the concept of maximum entropy, the mean and maximum channel velocities were related by a constant ratio, Ω, using data from three cross-channel surveys. This relationship was then used to estimate the freshwater discharge at the mouth based on the maximum velocity obtained from the detided ADCP velocity profiles. This approach was possible because the ADCP was deployed near the position of maximum current velocity, that is, over the deepest part of the channel. The results show good correspondence with observations, indicating that the entropy model can complete or substitute the records from upstream gauged stations that do not include the contribution from downstream tributaries. A Multilayer Perceptron neural net (MLP) based on the entropy approach was then implemented with the purpose of estimating the discharge when Ω is unknown. This latter analysis showsthat the relationship between maximum velocity and discharge is quasi-stationary. Consequently, the MLP can successfully estimate freshwater runoff if the training data represent all statistical properties of the river discharge dynamics. The results also indicate that Ω may vary not only with concomitant hydrographic conditions, but also with the recent (i.e., several days prior) discharge magnitude.

Improved marine‐derived POM availability and increased pH related to freshwater influence in an inland sea

Rapid changes, including warming and freshening, are occurring in coastal marine ecosystems worldwide. These environmental changes have the potential to alter ecosystem energetics by influencing availability of food sources and organism physiology. We investigated the influence of oceanographic variability on food availability and quality to benthic and pelagic suspension-feeders using detailed observations of phytoplankton, particulate organic matter (POM) detritus, and diverse biomarkers (fatty acids and carbon, nitrogen, and sulfur stable isotopes) along a salinity gradient in the San Juan Archipelago, Washington, U.S.A. We tested the hypothesis that freshwater input from riverine discharge would cause significant changes to oceanographic conditions and reduce food quality (indicated by essential fatty acids; EFA), owing to greater contribution of terrestrial organic matter. Contrary to our expectations, availability of high-quality marine-derived POM increased with freshwater input (reduced salinity). Phytoplankton biomass and biomarker composition responded to oceanographic change similarly across tidal and seasonal scales. Using a meta-analysis spanning a range of spatial and temporal scales, we found that chlorophyll a, temperature, dissolved oxygen (DO) and pH were consistently and significantly higher at reduced salinity. The increase of DO and pH corresponding to higher phytoplankton biomass in low salinity water signifies an important feedback of biological activity on seawater chemistry. This analysis supports the use of salinity as an indicator of processes controlling food availability and oceanographic conditions in this region. Collectively, these results highlight the importance of ecosystem connectivity in coastal environments and produce hypotheses for expected changes related to altered river discharge dynamics.

Reconstructing historical changes in phosphorus inputs to rivers from point and nonpoint sources in a rapidly developing watershed in eastern China, 1980–2010

Quantifying point (PS) and nonpoint source (NPS) phosphorus inputs to rivers is critical for developing effective watershed remediation strategies. This study reconstructed PS and NPS total phosphorus (TP) inputs to the Yongan River in eastern China in 1980–2010 using a load apportionment model (LAM) from paired riverine TP concentrations and river discharge records. Based on the fundamental hydrological differences between PS and NPS pollution, the LAM statistically quantified their individual inputs as a power-law function of river discharge. The LAM-estimated monthly/annual riverine TP loads were in good agreement with results derived from a regression model, Load Estimator (LOADEST). The annual TP load increased from 18.4 to 357.0Mgyr−1 between 1980 and 2010. The PS input contributed 7–45% of annual total TP load and increased 23-fold, consistent with a 20-fold increase in flow-adjusted average chloride concentration during the low flow regime (a proxy for wastewater inputs), as well as measured increases in population, poultry, and industrial production. Inferring from observed TP and chloride ratios, as well as total suspended solids (TSS) and river discharge dynamics, temporally retained P load within the river during the low flow regime was estimated to contribute 18–65% of the annual PS input load. NPS inputs consistently dominated the annual riverine TP load (55–93%) and increased 19-fold, consistent with the strong correlation between riverine TP and TSS concentrations, increasing developed land area, improved agricultural drainage systems, and phosphorus accumulation in agricultural soils. Based on our analysis, TP pollution control strategies should be preferentially directed at reductions in NPS loads, especially during summer high-flow periods when the greatest eutrophication risk occurs.

Calibration of satellite measurements of river discharge using a global hydrology model

► Orbital microwave sensors can monitor daily river discharge changes. ► Calibration of remote sensing signal to discharge units is necessary. ► Hydrological modeling is evaluated as a method to calibrate the remote sensing. ► Model-based calibration produces satisfactory results when compared to ground-based information. ► Independent river discharge information can now be obtained by use of model-calibrated remote sensing. Measurements of river discharge and watershed runoff are essential to water resources management, efficient hydropower generation, accurate flood prediction and control, and improved understanding of the global water cycle. Previous work demonstrates that orbital remote sensing can measure river discharge variation in a manner closely analogous to its measurement at ground stations, and using reach flow surface area instead of stage as the discharge estimator. For international measurements, hydrological modeling can, in principle, be used to provide the needed calibration of sensor data to discharge. The present study tests this approach and investigates the accuracy of the results. We analyze six sites within the US where gauging station, satellite measurements, and WBM model results are all available. Knowledge is thereby gained concerning how accurately satellite sensors can measure discharge, if the signal is calibrated only from global modeling results without any ground-based information. The calibration (rating) equations obtained for the remote sensing signal are similar, whether based on gauging station or on model information: r 2 correlation coefficients for least squares fits at one example site (#524; White River, Indiana) are both .66 ( n = 144, comparing monthly daily maxima, minima, and mean, 2003–2006). Space-based 4-day mean discharge values for this site when using the model calibration are accurate to within ±67% on the average ( n = 1824; largest percent errors occur at low discharges), and annual total runoff is accurate to ±9%, 2003–2008. Comparison of gauging station versus modeled discharge commonly indicates a small positive model bias; the observed errors of satellite-observed annual runoff are also positive and could be improved by bias removal from the rating curves. Also, analysis of a large flood event, along the Indus River in 2010, shows that the model does not capture flood wave attenuation by overbank flow, and thus predicts faster flood wave celerity and higher peak discharge than was measured by the remote sensing. The incorporation of overbank processes would improve discharge estimation via modeling, and also facilitate more accurate satellite-based measurement of peak discharge. The analysis shows that existing and planned microwave sensors can usefully characterize global river discharge dynamics, and that water balance model-based rating curves provide acceptable calibration of remote sensing signal to discharge.

Aquatic metabolism short-term response to the flood pulse in a Mediterranean floodplain

The pulsing of river discharge affects biodiversity and productivity of whole river–floodplain ecosystems, triggering the transport, storage and processing of carbon. In this study we investigate the short-term changes in water chemistry and net pelagic metabolism (NEP) in two floodplain lakes in response to a flood pulse. The two oxbow lakes investigated in the floodplain of the Mediterranean Ebro River (NE Spain) showed a clear temporal shift in their metabolic balance, controlled by the river discharge and associated changes in water physical and chemical characteristics. Water chemistry (turbidity, water organic matter, chlorophyll a and nutrients concentration) returned to pre-flood values after 4 days, highlighting the resilience of the ecosystem to flood pulses. Lake NEP was depressed before and during floods to a minimum of −34 mg O2 m−3 h−1, and increased after the flood pulse to a maximum of +463 mg O2 m−3 h−1. The phytoplankton assemblage showed before and after floods a replacement of autotrophic species (e.g. Chlorophyceans) by mixotrophic organisms (e.g. Cryptophyceans, Euglenoids). A linear mixed effects model identified abiotic factors, particularly temperature and river discharge, as significant predictors of the net aquatic metabolism and community respiration during flood conditions. Our results suggest that the role of the Ebro floodplain lakes as sources or sinks of C is complex and relative to the time scale investigated, depending strongly on the river discharge dynamics and the transport of limiting nutrients (phosphorus).

How is the impact of climate change on river flow regimes related to the impact on mean annual runoff? A global-scale analysis

To assess the impact of climate change on freshwater resources, change in mean annual runoff (MAR) is only a first indicator. In addition, it is necessary to analyze changes of river flow regimes, i.e. changes in the temporal dynamics of river discharge, as these are important for the well-being of humans (e.g. with respect to water supply) and freshwater-dependent biota (e.g. with respect to habitat availability). Therefore, we investigated, in a global-scale hydrological modeling study, the relation between climate-induced changes of MAR and changes of a number of river flow regime indicators, including mean river discharge, statistical low and high flows, and mean seasonal discharge. In addition, we identified, for the first time at the global scale, where flow regime shifts from perennial to intermittent flow regimes (or vice versa) may occur due to climate change. Climate-induced changes of all considered river flow regime indicators (except seasonal river flow changes) broadly follow the spatial pattern of MAR changes. The differences among the computed changes of MAR due to the application of the two climate models are larger than the differences between the change of MAR and the change of the diverse river flow indicators for one climate model. At the sub-basin and grid cell scales, however, there are significant differences between the changes of MAR, mean annual river discharge, and low and high flows. Low flows are projected to be more than halved by the 2050s in almost twice the area as compared to MAR. Similarly, northern hemisphere summer flows decrease more strongly than MAR. Differences between the high emissions scenario A2 (with emissions of 25 Gt C yr−1 in the 2050s) and the low emissions scenario B2 (16 Gt C yr−1) are generally small as compared to the differences due to the two climate models. The benefits of avoided emissions are, however, significant in those areas where flows are projected to be more than halved due to climate change. If emissions were constrained to the B2 scenario, the area with ecologically relevant flow regime shifts would be reduced to 5.4%–6.7% of the global land area as compared to 6.3%–7.0% in A2. In particular, under the B2 scenario, fewer rivers will change from perennial to intermittent (or transitional) river flows.

Groundwater–surface water interactions in a North German lowland floodplain – Implications for the river discharge dynamics and riparian water balance

Water balance and groundwater dynamics of a floodplain catchment in the Northeast German lowlands are investigated with consideration of the variable interactions between the riparian groundwater and surface water. Based on experimental and numerical investigations, evidence is given for temporally and spatially variable exchange fluxes between groundwater and surface water, which have significant impact on the riparian water balance and groundwater recharge. A coupled soil water balance–groundwater model is used for the quantification of exchange fluxes across the groundwater–surface water interface and the groundwater recharge for nested catchments in a typical floodplain representative for the Central European lowland catchments. Simulation results indicate substantial exchange fluxes between groundwater and the river, which are subjected to intensive spatial and temporal variability. The intensities and also the directions of exchange fluxes in particular stream reaches are characterised by transient alterations. Groundwater–surface water interactions are found to control the groundwater recharge dynamics in the floodplain and outweigh the influence of vertical percolation and root water uptake. Although groundwater contributions from this river stretch represent only 1% of the annual total discharge within the river its impact is much higher during low flow conditions in summer when ca. 30% of the river runoff which is generated in the catchment is originated by groundwater discharge from the riparian zone along this river stretch. The results of this study show that groundwater–surface water interactions within the investigated riparian floodplain are far too complex for the traditional classification concept distinguishing between loosing and gaining sections. The temporally variable impact of groundwater–surface water interactions furthermore highlights the necessity to consider seasonal effects when assessing the significance floodplain processes and functions.

Integrating groundwater dynamics in regional hydrological modelling

The paper presents an integrated catchment model and a method with which it is possible to analyse local water table dynamics inside subbasins along with river flow on the regional scale. A simple but comprehensive mechanistic groundwater module coupled with the eco-hydrological model SWIM (Soil and Water Integrated Model), which integrates hydrological processes, vegetation, erosion and nutrient dynamics at the watershed scale, was used in the study. The reliability of the model results was tested under well defined boundary conditions by comparing the results with those from a two dimensional numeric groundwater model under steady-state and transient conditions as well as with observed data for two meso-scale basins, using contour maps of the long term mean water table, observed groundwater level data in wells and observed river discharge. Especially in lowland catchments, where the water table is relatively shallow, the dynamics of river discharge are mainly influenced by changes in groundwater contribution to river flow. However, a correct reproduction of river discharge by hydrological models does not guarantee the adequacy of simulated spatio-temporal dynamics of soil moisture, water fluxes and groundwater in the basin. But even though the primary purpose of distributed hydrological models is to reproduce river discharge and water fluxes in the entire catchment, they are often validated using only the observed river discharge at the basin outlet for comparisons. The additional use of groundwater observations for model validation can serve as a measure to overcome the problem. The study area is located in the lowland part of the Elbe river basin, which is representative for semi-humid landscapes in Europe, where water availability during the summer season is the main limiting factor for plant growth and crop yields. The importance of adequate reproduction of the groundwater dynamics is illustrated in an investigation of a decreasing trend in regional groundwater level.